Existing direct sequence spread spectrum cellular communication system protocols require base stations within the communication system to be time synchronized. For example, within a wireless communication system utilizing a Code Division Multiple Access (CDMA) system protocol, it is desirable to have all base stations within the communication system synchronized to +/-3 microseconds (.mu.s), and it is mandatory that all base stations within the communication system be synchronized to at least to +/-10 .mu.s. Synchronization occurs within a CDMA system as described in "Personal Station-Base Station Compatibility Requirements for 1.8 to 2.0 GHz Code Division Multiple Access (CDMA) Personal Communication Systems" (ANSI J-STD-008). In particular, all base stations reference a common CDMA system-wide time scale that uses Global Positioning System (GPS) time. All base stations utilize the same pseudo-noise (PN) spreading code, but with different time offsets. A remote unit uses a correlator to detect the presence of the PN code, and will detect all base stations in the geographic region if it searches the entire length of the single PN. In the described system, the base stations are offset from each other by integer multiples of 64 PN chips, thus allowing 512 unique offsets of the length 2.sup.15 or 32,768 chips. The base station which is in communication with a remote unit communicates the PN (time) offset of each neighboring base station which the remote unit should search. A major advantage in having all base stations within a communication system utilize a common system time, is that during handoff between base stations, a remote unit only need to look within a very small time window around the nominal PN offset to acquire a new base station.
In order to reduce system costs, some current CDMA developers are proposing that base stations within next-generation CDMA systems be unsynchronized. An example of an unsynchronized next-generation CDMA system is that proposed in "Coherent DS-CDMA: Promising Multiple Access For Wireless Multimedia Mobile Communications", F. Adachi, M. Sawahashi, T.Dohi, and K.Ohno, ISSSTA 1996, Mainz, Germany. In this proposal, each base station has a unique long code and a common short code. The base primarily transmits a product of the two codes, but at defined times will mask the long code and transmit only the short code. Thus a remote unit may search for the common short code and get a periodic strong match from a strong base station, and a periodic weaker match from a weaker base station. The proposal describes a process for the mobile to first detect short codes, then determine the long code timing, a long code group identification, and then the long code identification. A mobile in communication with one base station must continually search for the presence of nearby base stations using this same process, since base stations are unsynchronized and cannot communicate timing information to the remote units to reduce their search. Because of this, remote units handing off within an unsynchronized system will need to look within a larger time window to acquire a new base station. Since there currently exists no method to determine whether a communication system is synchronized or not, remote units handing off within next-generation CDMA systems will be required to search a larger time window when handing off, even if the base stations are time synchronized, resulting in unnecessarily long handoff times.
It is desirable to have synchronized base stations for fast handoff, yet allow the system to operate in geographic areas where accurate time synchronization is unavailable, or continue to operate when time synchronization fails. Therefore, a need exists for a method and apparatus for determining if a communication system is synchronized and adjusting a search window accordingly during handoff.